Pneumatically driven injection pumps are well known. Such pumps are particularly useful for injecting small amounts of liquids into vessels or flow lines at relatively high pressures. These pumps typically employ a low pressure air supply to drive a large diameter piston that coacts with a small diameter plunger to multiply the pressure delivered through the high pressure injection cylinder. The required supply gas pressure is determined by dividing the desired injection pressure by the published pump ratio. The flow of supply gas to the large diameter piston is controlled by a relay valve adapted to cycle the pump frequently enough so as to achieve the desired flow rate. Different relay valve configurations have been utilized by the manufacturers of the various conventional, commercially available pumps. These include valves employing mechanical switches, pressure shifted spool valves, and mechanical shifted spool valves. However, injection pumps employing each of these types of relay valves have experienced undesirably high failure rates. Such failures usually occur at the bottom end of the power stroke where the relay doesn't quite reset.
With the relays actuated by mechanical switches, problems have been encountered with spring failure. With the pressure shifted spool valve relays, O-ring problems sometimes cause the spool not to shift properly or completely in response to light pressure loads, resulting in stall. Stall occurs because the miniature bleeder valves that operate the spool partially exhaust. Similarly, with the mechanically shifted spool valve relays, stalling may occur as a result of mechanical failure or by a partial bleeding through the relay, thereby robbing the air piston of its driving force.
Conventional air driven injection pumps are commercially available from suppliers such as Arrow Specialty Company of Tulsa, Oklahoma, Haskell, Inc., of Burbank, California, and Sprague Engineering of Gardena, California.